Float drums have evolved from crude sealed metal drum and blocks of styrofoam into hollow polyethylene shells which are precisely configured and designed to fit especially within intricate dock structures typically sold in kit form. An example of an early design of a hollow polyethylene float is disclosed in the Shuman patent noted above and marketed today by the assignee of the present invention under the "Dayton" brand name. My prior patent (U.S. Pat. No. 4,799,445) describes a current, state of the art polyethylene float and the integration of the float within sophisticated floating marine structures. Specifically, today's floats are constructed with rigidized circumscribing flanges for secure mounting to the dock structure; ribs secured to the underside of the flange for strengthening the side walls and the flange and spaced indentations which also strengthen the float walls while providing indentations for receiving dock structure members or, alternatively, for interlocking the float drums one on top of the other or side-by-side.
Polyethylene float drums are typically formed as hollow shells in a process generally described in the trade as "twin sheet forming". The process is generally described in the process patents noted above in subparagraph (d) and incorporated by reference herein. The floats are then marketed either as hollow, air filled floats or the floats are filled with a flotation material. In the latter instance, a hole is provided in the drum and a urethane foam is injected into the float, expanded and cured. As noted in the Heinrich patent, the urethane is usually introduced into the float while the float is still confined to prevent bulging. When the urethane is cured, the filled float provides a somewhat rigid foam reinforcement for polyethylene walls thus adding strength to the float while also preventing leakage of water into the float if the polyethylene walls are accidentally or maliciously punctured.
While the foam filled polyethylene floats have inherent advantages over hollow or air filled floats, in practice problems have been encountered. First, whether or not the polyethylene float is retained in the forming die or in some other fixture while the urethane foam is injected, the configuration of present day float drums is such that the foam, when cured, does not expand against all the walls of the float. Spaces between the foam and the polyethylene walls inevitably occur with the result that the wall can flex until encountering the cured foam. While the float would not sink even if the walls could flex to the point of rupture, the strength or rigidity of the float is obviously less than what is otherwise possible. Another problem which has been encountered is that, inherent in the twin sheet forming process, shrinkage does occur when the polyester dries. Because of the especially configured shape of the drum which prevents the form from uniformly contorting the entire wall surface, the walls can become bowed and sometimes significantly so. The bowed or distorted walls which are not corrected by the foam fill make the float aesthetically unattractive and could interfere somewhat with the usefulness of the indentation features, etc.